In an energy power harvesting technique that stores, and utilizes for a sensor or wireless transmission electric power, electric power generated from minute energy such as sunlight, oscillation, or a body temperature of a human being or an animal, a safe and highly reliable secondary battery under any terrestrial environment is desired. A secondary battery that has widely been utilized at present uses an organic electrolyte solution that is a liquid, and may cause a positive electrode active substance to be degraded due to multiple use thereof and a capacitance of the battery to be lowered or may cause an organic electrolyte in the secondary battery to be ignited and take fire due to a battery short circuit caused by formation of a dendrite. Therefore, it is not preferable to use a current secondary battery that uses an organic electrolyte solution, in, for example, energy power harvesting wherein utilization for 10 years or more is expected, from the viewpoint of reliability or safety thereof.
On the other hand, attention is being paid to an all-solid lithium secondary battery that does not use an organic electrolyte and is entirely formed of solid component materials, because there is not a risk of solution leaking, firing, or the like and a cycle characteristic thereof is also excellent. For a lithium-ionic conductor that is a solid electrolyte to be used for such an all-solid lithium secondary battery, there is provided an oxide type, a sulfide type, or the like. For an oxide type, there is provided a LISICON (LIthium Superlonic CONductor) structure based on Li3PO4 or Li4GeO4, a NASICON (Na Superlonic CONductor) structure based on an sodium-ionic conductor, a LiLaZrO garnet structure, a perovskite structure such as an LLTO, or the like. Furthermore, for a sulfide type, there is provided Li10GeP2S11, Li7P3S11, or the like.
Meanwhile, although LiCoO2 (theoretical capacitance: 137 mAh/g) has widely spread for a positive electrode material of a secondary battery, Electrochimica Acta 56 (2011) 2201-2205 discloses, in a recent year, Li9V3(P2O7)3(PO4)2 (theoretical capacitance: 173 mAh/g) that exceeds this. This Li9V3(P2O7)3(PO4)2 utilizes an oxidation reduction reaction of V3+→V5+, and has a theoretical capacitance that is 1.2 times as large as that of LiCoO2.
However, Li9V3(P2O7)3(PO4)2 is a material with a crystal structure that is greatly different from that of conventional LiCoO2 that has been used as a positive electrode material, and an ionic conductor has not existed that is a solid electrolyte suitable for Li9V3(P2O7)3(PO4)2.